Patent document 1: JP-A-2004-136831
Recently, an electric vehicle (an EV vehicle), a hybrid vehicle (a HV vehicle), or the like only generates small noise according to a structural characteristic. A pedestrian may not notice approach of the vehicle. Therefore, so as to improve a recognition of the pedestrian that the vehicle exists near the pedestrian or the like, a vehicle approach warning apparatus generating a warning sound (a sound effect) may be mounted to the vehicle.
For example, patent document 1 discloses a vehicle approach warning apparatus in which a memory stores a sound source of a warning sound such as an engine start-up sound, a traveling-start sound, a low speed traveling sound similar to a sound generated when a vehicle travels at low speed, a medium speed traveling sound similar to a sound generated when the vehicle travels at medium speed, and a high speed traveling sound similar to a sound generated when the vehicle travels at high speed. The vehicle approach warning apparatus selects and outputs a different warning sound according to a vehicle speed or the like. Patent document 1 discloses that the warning sound is an impressive sound including a lot of a low frequency component at the time of low speed traveling. At the time of high speed traveling, the warning sound is a shrill sound including a high frequency component, so that a pedestrian may easily recognize the vehicle.
Since the apparatus described in patent document 1 selects and outputs different kinds of warning sounds according to vehicle speed or the like, a user may have uncomfortable feeling when the warning sound is discontinuously changed according to the vehicle speed. Many kinds of sound sources may be prepared, and the sound sources may be selected according to vehicle speed or the like, so that the warning sound is gradually changed so as to reduce the uncomfortable feeling. However, in the above case, the apparatus requires a large capacity memory. A cost for the apparatus may increase.